The present invention relates to microcellular foams made from thermosets, and also to a process for their production. It further relates to partially crosslinked thermosets which comprise blowing agents and are intermediates in the novel process.
The foams most widely used hitherto have relatively large cells of the order of size of from about 50 to 1000 xcexcm and have polyurethanes or thermoplastics as matrix material. The large cells bring about a reduction in the quotient calculated from (e.g. tensile) strength and density to below the value of the corresponding compact material, i.e. as porosity increases strength reduces more rapidly than density. For many applications, however, the opposite effect would be desirable, since this would enable genuine savings of material to be made or greater forces to be withstood using the same amount of starting material. It is known that reducing the cell size to values of the order of size of 10 xcexcm or below can in some cases dramatically increase the strength of foams (U.S. Pat. No. 4,473,665, U.S. Pat. No. 5,160,674). The term xe2x80x9cmicrocellular foamsxe2x80x9d has become established for foams of this type. These known microcellular foams are based on thermoplastic matrix materials and the processes for producing them can only be used for materials of this type. However, significant disadvantages are associated with the use of thermoplastic matrix materials. Some of these are associated with the process and others relate to the properties of the resultant foams. A process-related disadvantage is the need for high pressures during moulding, due to the high viscosity of the polymer melts. This requires the use of extruders. However, the most significant disadvantage is that the thermoplastic foams have limited heat resistance. In contrast, thermosets can be processed at low pressures since with these the final polymerization or crosslinking does not take place until after moulding. In addition, their maximum working temperature is higher, reaching up to about 400xc2x0 C. in the case of polytriazine resins, for example. Their three-dimensional crosslinking also gives them higher strengths than most thermoplastics, although they are more brittle than these in the compact state. There are also some known foams based on thermosets, but these have the large cells mentioned at the outsetxe2x80x94with the resultant poor strength valuesxe2x80x94and are moreover mostly of open-cell type. The object of the present invention was therefore to provide microcellular foams made from thermosets which combine the known good properties of thermoset matrix materials with the advantageous effects of the microcellular state.
According to the invention, this object is achieved by the foams according to the microcellular rigid foam of the invention.
It has been found that a suitable production process can give microcellular rigid foams with a matrix made from crosslinked thermosets and with an apparent density of from 20 to 95% of the density of the compact matrix material. xe2x80x9cMicrocellularxe2x80x9d here means an average cell diameter of less than 50 xcexcm, preferably less than 25 xcexcm. The novel foams are predominantly of closed-cell type.
The matrix material of the novel microcellular foams is preferably selected from the group consisting of polytriazine resins, epoxy resins, phenolic resins, polyimide resins and combinations or blends of two or more of these resins.
Particular preference is given to matrix materials made from polytriazine resins obtained by cyclotrimerizing one or more aromatic cyanates.
Very particular preference is given to matrix materials made from polytriazine resin obtained by cyclotrimerizing an aromatic cyanate prepared by cyanating a novolak. These aromatic cyanates are obtainable, for example, from Lonza AG as Primaset(trademark) PT Resins.
The novel microcellular foams may be produced by impregnating the prepolymerized or precrosslinked thermosets, at elevated pressure and elevated temperature, with a physical blowing agent, then expanding the materials at a lower pressure and, if desired, higher temperature and finally fully curing the materials, where the temperature during the expansion procedure and curing procedure is always below the glass transition point of the material at that juncture. The prepolymerization or precrosslinking here may take place in an entirely separate process step or immediately prior to the impregnation with the blowing agent. The impregnated matrix material may be cooled under pressure and thus be converted into a storage-stable state with physically dissolved blowing agent, so that the expansion takes place in a step which is spatially and/or chronologically separate. Alternatively, the expansion may take place immediately after impregnation. An important factor for the success of the novel process is the correct degree of prepolymerization prior to impregnation. If the degree of polymerization is too small a conventional foam is obtained, and if prepolymerization is too extensive no foam at all is formed, and all that happens is that the material cracks and to a greater or lesser extent is blown apart. The correct conditions for the prepolymerization depend on the thermoset used and in some cases have to be determined by experiments. It is also important that during the expansion and the curing the temperature always remains below the glass transition point. The glass transition point is lowered by the dissolved blowing agent, which acts as xe2x80x9cplasticizerxe2x80x9d, and rises during the expansion and the curing.
Examples of suitable blowing agents are low-molecular-weight hydrocarbons, in particular C1-6 alkanes or C3-6 cycloalkanes, low-molecular-weight halogenated hydrocarbons and carbon dioxide. An important factor is the presence of a degree of mutual solubility, and substances with no solvent properties, such as nitrogen or noble gases, are unsuitable.
The impregnation with the blowing agent is preferably carried out at a pressure and temperature in the region of the critical point of the blowing agent, or thereabove.
Carbon dioxide is particularly preferred as blowing agent. The examples below further clarify the invention which should not be regarded as limiting.